LTE DIN Rail Analog Transmitters with Ethernet Communication and Analog Outputs for RTD Temperature Applications

LTE RTD Temperature Transmitter Frequently Asked Questions

Why does Pt100 DIN (alpha 0.003850) carry a documented ±0.03°C accuracy figure while Pt100 ANSI (alpha 0.003902) is documented at ±0.04°C, despite both being 100Ω platinum RTDs?

Documented specification lists these as genuinely separate accuracy figures tied to two different calibration standards — IEC 751 (IPTS-68) for the DIN alpha curve versus NIST Monograph 126 for the ANSI alpha curve — since these are documented as distinct calibration references with their own characterization data, the resulting achievable accuracy figures for each curve are documented separately rather than being identical simply because both use 100Ω platinum elements.

Why does sensor lead resistance tempco improve by roughly three orders of magnitude between 2-wire (10 mdeg/Ω/deg) and 3-/4-wire (10 µdeg/Ω/deg) configurations?

Documented figures specifically quantify this difference in lead-resistance-driven temperature error per ohm per degree — since 3- and 4-wire configurations are documented as actively compensating for or eliminating lead resistance effects (through excitation/sense lead separation), while 2-wire configuration is documented as only compensating for the initial resistance value measured at setup (not its later temperature-driven drift), the residual documented tempco for 2-wire is substantially larger, reflecting the lead wire's own resistance change with ambient temperature going uncompensated.

Does the documented 3-wire compensation technique work correctly if the two excitation leads have genuinely different lengths or gauges?

No — documented description specifically qualifies the 3-wire compensation technique as effective "if the two excitation leads are identical," meaning the method's accuracy depends on this stated assumption; if the two excitation leads genuinely differ in resistance (due to length, gauge, splices, or connection quality), the documented subtraction of "twice one excitation lead's voltage drop" would not perfectly cancel the actual combined lead resistance, since the calculation assumes both leads' resistance is the same as the one it's actually measuring.

Why does the 2-wire hookup method require "shorting out the RTD during transmitter setup" rather than compensating for lead resistance automatically like the 3-wire method?

Documented description specifically explains that 2-wire hookup senses the combined voltage drop across the RTD and both lead wires together, with no separate, independent way to isolate just the lead wire contribution during normal operation; shorting the RTD during setup is documented as the specific method used to capture just the lead wire resistance in isolation at that moment, which is then subtracted — but since this is a one-time setup measurement, documented limitation notes any later lead resistance change from ambient temperature won't be automatically compensated the way it is in 3- and 4-wire configurations.

Does the documented user calibration feature (multiplier plus offset) allow correcting for an RTD with a genuinely different alpha curve than the transmitter is configured for?

The page documents user calibration specifically as addressing RTDs "whose resistance is different than nominal at 0°C," which is consistent with correcting a manufacturing tolerance or small resistance offset at the reference point — this is documented as a distinct concern from selecting the correct underlying RTD type/alpha curve at time of order (P385 for DIN alpha, P392 for ANSI alpha), so a genuinely mismatched alpha curve would need the correct P385/P392 signal input selected, rather than being correctable solely through the documented user calibration multiplier/offset.

Why does copper (Cu10) RTD excitation current (5.0 mA) differ so substantially from the 196 µA used for Pt100 and Ni120?

Documented specification lists these substantially different excitation currents without detailing the underlying circuit reasoning — this is consistent with Cu10's documented much lower base resistance (9.035Ω at 0°C, versus 100Ω for Pt100 and 120Ω for Ni120) requiring proportionally more excitation current to develop a comparably measurable voltage signal across the sensor for the same signal conditioning circuitry to process accurately.

Does this LTE RTD transmitter's documented Modbus TCP-only protocol limit compatibility compared to the RS232/RS485 LT Series RTD variant?

Yes — this page documents Modbus TCP specifically as the supported Ethernet Data I/O protocol at digital address 247, while the LT Series serial variant is documented elsewhere as separately supporting Modbus RTU/ASCII and Laurel Custom ASCII; a control system needing a protocol other than Modbus TCP would need to reference the LT Series serial variant rather than this LTE Ethernet variant.

Does the documented note that the same signal conditioner board supports both RTD types and resistance measurement mean an RTD transmitter can be reconfigured for general resistance measurement without new hardware?

Documented note specifically states the board "can be user configured for all RTD types listed... as well as for resistance measurement," which is consistent with resistance measurement being an available configuration option on the same underlying board hardware — though the page itself doesn't detail whether this reconfiguration is purely a software/setup change or also involves jumper settings, similar to how DC voltmeter versus ammeter operation is documented as jumper-selected on other LT/LTE transmitters.

Does zero tempco (±0.03 deg/deg) and span tempco (±0.003% of reading/°C) represent the same underlying drift mechanism?

No — these are documented as two separate specifications describing different aspects of temperature-driven measurement drift: zero tempco describes drift in the reading at the zero/reference point, while span tempco describes drift in the overall scaling of readings across the measurement range as ambient temperature changes; both contribute to total measurement drift but through documented, physically distinct mechanisms.

Does selecting a wider RTD range (such as the full -202°C to +850°C Pt100 DIN span) change the transmitter's documented ±0.01% of reading accuracy component?

No — documented accuracy is expressed as a fixed error figure (such as ±0.03°C for Pt100 DIN) plus a separate ±0.01% of reading component; the percentage-of-reading portion is documented as applying proportionally regardless of the specific span selected, while the fixed error figure remains constant, meaning total accuracy at any given actual temperature reading follows the same documented formula whether a wide or narrow input span is configured.

3-Wire RTD Lead Wire Matching Questions From the Field

Why does even a small amount of lead resistance cause a disproportionately large temperature reading error in a Pt100 RTD?

Documented explanation specifically notes a 100-ohm platinum RTD changes resistance by only about 0.385 ohms per degree Celsius — since this per-degree resistance change is so small, documented calculation shows a lead resistance of just 2 ohms can translate into roughly a 5°C measurement error, illustrating how the RTD's inherently small resistance-per-degree sensitivity makes it disproportionately vulnerable to any uncompensated lead resistance.

What real-world conditions commonly break the "identical lead resistance" assumption that 3-wire compensation depends on?

Documented field examples specifically cite a splice or repair performed on one conductor but not the other, a field repair using mismatched wire gauge because it was what was available on hand, and a corroded or loose terminal adding resistance on just one side of the pair — these are documented as genuine, common real-world causes of the two "identical" excitation leads becoming measurably mismatched over an installation's service life.

Is there a documented quick field test to verify whether a 3-wire RTD installation is correctly wired?

Yes — documented field test specifically involves swapping any two of the three lead wires at the controller terminals; if the temperature reading changes significantly after the swap, documented guidance identifies this as indicating a wiring fault, since a properly wired and matched 3-wire RTD installation is documented as showing the same reading regardless of which two wires are swapped.

Why is a 4-wire RTD configuration documented as being immune to lead resistance mismatch in a way 3-wire configurations are not?

Documented explanation specifically attributes this to the 4-wire configuration's completely separate voltage-sensing wire pair carrying essentially zero current, due to the high-impedance nature of a voltage measurement input — since negligible current flows through those sensing wires, their resistance has essentially zero effect on the reading regardless of length or mismatch, which documented analysis contrasts directly with 3-wire's dependence on the "leads are identical" assumption.

Does accidentally jumpering or shorting the two excitation leads together at the transmitter terminals of a properly matched 3-wire RTD actually introduce new error, or does it simply reduce accuracy back toward 2-wire levels?

It genuinely introduces new error, documented as worse than simply reverting to 2-wire performance — one documented worked example specifically shows a normal, well-matched 3-wire installation producing 0.00°F of lead-related error, while jumpering the two excitation leads at the transmitter produces a documented +0.94°F error purely from that jumper, illustrating the jumper doesn't just remove the 3-wire compensation benefit but actively creates a new, uncompensated resistance path.

Does using the same wire gauge and type throughout a 3-wire RTD installation genuinely improve real-world accuracy, or is this mainly a theoretical best practice?

It's documented as a genuine, practical best practice with a measurable accuracy benefit — documented guidance specifically states using the same type of wire on all three RTD lead wires makes a 3-wire installation "as accurate as possible," directly tying the practical wire-selection decision to the underlying mathematical assumption (identical lead resistance) that the entire 3-wire compensation technique depends on.

Is there a documented typical real-world accuracy difference between 2-wire and 3-wire RTD configurations, expressed as a concrete number rather than just "more or less accurate"?

Yes — one documented comparison specifically cites 3-wire RTDs as typically achieving ±0.5 to 1°C accuracy compared to ±1 to 5°C for 2-wire configurations, assuming lead wires up to 100 feet in length; this documented range gives a concrete sense of the real accuracy gap between the two configurations under a common practical installation length.

Does grounding the cable shield at both the sensor end and the controller end of a 3-wire RTD installation improve noise rejection?

No — documented guidance specifically warns against grounding both ends of a shielded RTD cable, since doing so creates a ground loop that introduces noise rather than rejecting it; documented best practice specifically recommends grounding the shield only at the controller or cabinet side, leaving the sensor end of the shield unconnected.